1. Field of the Invention
The present invention concerns an amplifier device of the type having a radio-frequency power amplifier and a voltage supply device for the radio-frequency power amplifier.
2. Description of the Prior Art
Amplifier devices of the above type are generally known and are used for a wide variety of applications. One possible application is the supply of an RF transmission coil of a magnetic resonance system with electrical energy.
In medical uses of magnetic resonance systems, the transmission coils must be supplied with pulsed currents over a large dynamic range. The current pulses exhibit essentially the same energies among one another, but exhibit power requirements significantly deviating from one another. This has the result that the radio-frequency power amplifier must emit pulses of long duration in the lower power range in which the radio-frequency power amplifier exhibits a poorer degree of efficiency. This is a disadvantage with regard to the power loss that occurs in the radio-frequency power amplifier. The requirements for reliability of the radio-frequency power amplifier and the cost-effectiveness of the operation require the radio-frequency power amplifier to be operated efficiently in the entire dynamic range.
One possibility to achieve a better efficiency is to dynamically optimize the supply voltage of the end stage of the radio-frequency power amplifier with regard to the currently required output power. This solution delivers a nearly perfect result when only the radio-frequency power amplifier is considered, but it normally leads to substantially increased power losses in the voltage supply device for the radio-frequency power amplifier.
In medical uses of magnetic resonance systems, the radio-frequency antenna is operated in a pulsed power mode in which it requires a significant maximum current at its highest operating point. This current requirement limits the possibility to use a high-efficiency switched voltage supply. Furthermore, a large capacitance battery is required in order to store the required charge. The charge is typically stored by a number of capacitors connected in parallel that are dimensioned for the maximum voltage of the radio-frequency power amplifier. The maximum voltage is calculated as the minimum voltage that is required in order to correctly operate the radio-frequency power amplifier plus the voltage decrease due to the capacitor discharge when radio-frequency pulses are required. In practical embodiments this means that the capacitors must be designed for very high voltages and must be able to store a large charge. They therefore require a large installation space.
A device for dimming internal monitoring lights of military vehicles is known from DE 35 34 930 A1. In this device an electrical monitoring light circuit arrangement with at least one Zener diode and a relay connected in parallel with the Zener diode is used. The circuit arrangement is electrically connected with a hooded light switch such that, given an activated ignition of the vehicle and given switching to “hooded light off”, the relay coil changes to or draws a predetermined maximum level upon bridging of the associated Zener diode for the purpose of dimming the monitoring lights.
A power amplifier with at least two end stages connected in series at the output side is known from DE 198 12 069 A1, wherein each of the end stages has a power bridging circuit in order to generate an end stage voltage at each stage by pulse width modulation according to an end stage switching clock. The output voltage of the power amplifier is a combination of the end stage voltages. The end stage switching clocks of the end stages are offset from one another in order to increase the effective switching frequency of the output voltage.